A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The European patent application EP 0 957 402 A2, which is incorporated in the present application by reference, describes a lithographic apparatus, in which a projection system is separated from a substrate table by an intervening space. The intervening space can be at least partially evacuated. The intervening space contains a hollow tube that is continually flushed by a flow of gas to reduce cross-contamination between the substrate table and the projection system. The gas does not substantially absorb EUV radiation. According to EP 0 957 402 A2, the gas is Ar or Kr. During use, radiation is directed through the tube, from the projection system to a substrate, which is held by the substrate table. An advantage of the use of argon is, that the ‘removability’ or ‘pumpability’ of argon is relatively good. For example, in typical vacuum pumps used in lithographic apparatus, such as turbomolecular pumps, the pumping speed for argon (relative molecular mass 40) is relatively large compared to the pumping speed for very light gases, such as hydrogen (relative molecular mass 2), or very heavy gases like Xenon (relative molecular mass 131).
International application WO 01/84241 A1, which is also incorporated herein by reference, describes a lithographic apparatus and method, that includes a non-contact seal using a purge gas. Therein, a purged optical path between an optical source surface and an optical target surface is provided, as well as relative movement between the optical source surface and the optical target surface. A purge gas of a controlled purity is used.